One extracorporeal circuit which has heretofore been used is an oxygenator circuit system which substitutes for the functions of the heart and lung during, for example, cardiotomy surgery. Referring to FIG. 5, the oxygenator circuit system 100 generally includes an oxygenator 1, a heat exchanger 50, a blood reservoir 31, a blood filter 70, a blood line 90 interconnecting the foregoing units and a human body 94, and a pump 95.
Most of the oxygenators used are membrane oxygenators. The membrane oxygenator has a gas-exchange membrane disposed in a housing such that gas exchange is carried out by passing blood over one surface of the gas-exchange membrane and an oxygen-containing gas over the other surface of the membrane. Most of the commonly used gas-exchange membranes are hydrophobic membranes including hydrophobic porous membranes formed of polypropylene, polyethylene or the like and diffusion membranes formed of silicone rubber, etc.
In use, a priming operation is carried out to clean the interior of the membrane oxygenator and remove air therefrom before blood is passed through the oxygenator. It is difficult to completely remove air during the priming operation. Particularly with a hollow fiber oxygenator using hydrophobic porous hollow fibers as the gas-exchange membrane, there occurs an air blocking phenomenon whereby air is taken into the fluid side from the gas side so that unescapable gas will stagnate between hollow fiber membranes on the fluid side. As a result, those portions of hollow fiber membranes in contact with the stagnating gas do not contact blood, negating the effective use of hollow fiber membranes. Thus the oxygenator sometimes fails to exert its full gas-exchange ability. The blood filter functions to remove foreign matter and bubbles from the gas-exchaged blood on the way back to the human body. The blood filter also uses a hydrophobic membrane. It is thus difficult for the priming operation to completely remove air for the same reason as with the aforementioned membrane oxygenator. Particularly, the blood filter has a problem whereby air is left on the surface of a hydrophobic membrane to reduce the effective surface area of the hydrophobic membrane, eventually increasing the pressure loss across the blood filter.
Besides the membrane oxygenator, heat exchanger, and blood filter, blood tubes used for fluid communication of these units to the human body are generally formed of flexible synthetic resins such as vinyl chloride and silicone rubber. The aforementioned priming operation is carried out throughout the tubes as well as the oxygenator and blood filter. Since the blood tubes are formed of the above-mentioned material, their inside surface is hydrophobic. It is thus difficult to remove fine bubbles adhered to the inside surface of the tubes by the priming operation. Upon blood circulation, such bubbles will gradually enter the blood, causing blood foaming.
Further, the membrane oxygenator and blood filter include many portions formed of hydrophobic resin in addition to their membranes. The same applies to other units involved in the oxygenator circuit, for example, a blood reservoir and a heat exchanger. For example, housings of the membrane oxygenator, blood filter, blood reservoir, and heat exchanger are generally formed of hydrophobic resins such as polycarbonate, polystyrene, MBS, and polypropylene. The housings thus have many blood contact portions of hydrophobic material. It is difficult to remove fine bubbles adhered to the inside surface of the blood contact portions by the priming operation as in the case of the inside surface of the blood tubes mentioned above. This causes the introduction of bubbles into blood upon blood circulation.
An object of the present invention is to provide a medical instrument in which air removal can be readily completed by a priming operation prior to blood circulation, leaving few bubbles adhered, as well as to provide a method for fabricating the same.